Tire Pressure – Achieving the Magic Number

By Michael Seinberg

One of the most important steps in having a fun and safe bike ride is making sure your bike’s tire pressure is properly set and maintained. This may sound like a no-brainer to experienced riders, but even the pros get this one wrong now and again. The truth is, keeping your tires properly inflated on a bike is also a huge safety issue because unlike a car, which floats along on four huge 32 psi (pounds per square inch) balloons, bikes have two comparatively skinny tires, and that means you need to pay attention to them far more than you do in the car.

If your tires are underinflated, you run the risk of snakebite flats if you hit a square edge. You must expend far more power pedaling if your tires are underinflated too. If your tires are always underinflated, they wear out faster too. On the road, underinflated tires are also unpredictable in corners. In contrast, if you overinflate, you run the risk of a sudden blow out which can easily send you out of control. Overinflated tires will bounce you silly off-road and they won’t grip properly on or off-road.

To keep things properly inflated you’ll also need at least one pump if not several. A good floor pump that features a dial pressure gauge is a great first pump for most people. Figure on spending $25-75 for a nice quality unit. If you spend a fair bit of time in the saddle, you’ll likely want a frame pump that attaches to your bike, and can be used on the road or trail if you get a flat. You’ll also want to install a seat bag with a spare tube, tire levers and possibly a patch kit if you run tubeless tires. And make sure your pump is properly set up for your valve type. Most modern road bikes and many mountain bikes now run Presta valves (the skinny ones with the little nut at the top) and many kid bikes and some mountain bikes run Schrader valves (like the ones on your car). Most better pumps can be adapted to work on either.

So how do you know how much air you need? Let’s start at the beginning. All modern bike tires tell you how much pressure they need. That information is printed right on the sidewall. You’ll either see an exact figure or a range.

For road bikes you’re generally looking at upwards of 100 psi for 700 23-25c tires. Some go as high as 120 psi. But here’s the funny thing about road tires: they lose pressure fairly quickly too. If you pump up your tires to 110 psi on Monday and take a ride, then hang up the bike, the tires could easily be down to 80-90 psi by Friday. There’s no leak either, it’s just the nature of high-pressure rubber tubes and tires. So, your lesson with road tires is to check your pressure before every ride.

Most mountain bike tires run in a range that goes from 30-65 psi. This is because where you set your MTB pressure depends on several factors including your weight, the riding terrain, and the surface conditions. If you’re riding on soft sand, mud or loose soil, you want to go a bit on the lower side of the range, to allow the tires to compress a bit and grip better. If your ride includes a lot of hard surfaces, rocks, ruts and roots, you need more pressure but not too much as you’ll bounce rather than grip. The size of your tires also factors in. Wider tires don’t need to be pumped up as high as narrower tires. Even on the lower end, wider tires tend to have decent grip and handling.

The real key to finding your ideal off-road pressure is to experiment. Go to your usual trail and pump your tires up above what you might normally. Try a section you know well and see what you think of the ride. Drop each tire 5-10 psi and try again. Pretty soon you’ll see where you prefer things. After a while you’ll develop a sense of what pressures work for what conditions.

Hybrid tires are mostly like road tires only usually a little wider so, in many cases, the tire pressures are lower. My wife has tires on her hybrid that are around 700c x 35c and likes to be about 65 psi. I was running slightly narrower ones and mine liked to be around 80 psi. Also, hybrids can, in many cases, be used off-road so set your pressure accordingly. I have some 700c x 35c cyclocross tires on my hybrid bike and take it off-road on pretty gnarly trails, so I tend to drop the pressure down a bit from the recommended because if I went out on the rocks at 65 psi I’d be bounced off into the trees.

Kid bikes (12-20” tires) are pretty straightforward. Just pump them to the recommended pressure and check them about weekly. Your average 20” kid bike tire tends to go around 40 psi, and for kids just riding in the driveway or around the neighborhood, that’s fine. The exception is kids who race BMX or ride 24” mountain bikes and actually go off-road. Then, follow a more MTB approach and adjust based on terrain and conditions.

These guidelines are all basic and based on reading, riding, testing and experience. Technology keeps changing, so if you have a bike that doesn’t fit any of these standards, say a high-tech beast running tubeless tires on carbon fiber rims, then, by all means, check with the dealer you got the bike from or the manufacturer. The key is to maintain enough pressure to allow the tire to hold its desired shape and perform in the way it was designed to.

To put this into perspective, think about the following. I had to do some work on a 1940s English bike that ran 26 x 1 3/8” tires on rims that featured rim brakes actuated by rods, not cables and a fully enclosed chain. On the side the original tires said, “wartime standard” and “pump until hard.” We have come a long way!

Michael Seinberg (mseinber@nycap.rr.com) of Altamont is a professional bicycle mechanic, rider and ex-racer with more than 30 years of experience. But he’s still learning new things every day.